core: New encryption flags GPGME_ENCRYPT_ADD_RECP and _CHG_RECP.

* src/gpgme.h.in (GPGME_ENCRYPT_ADD_RECP, GPGME_ENCRYPT_CHG_RECP):
New flag values.
* src/engine-gpg.c (have_cmd_modify_recipients): New.
(gpg_encrypt): Check availability of the feature and prepare command.

* tests/run-encrypt.c (main): New options --add-recipients
and --change-recipients.
--

GnuPG-bug-id: 1825

5 files changed, 71 insertions, 3 deletions

diff --git a/NEWS b/NEWS
index fdca7ab1..e6d1467f 100644
--- a/NEWS
+++ b/NEWS
@@ -17,6 +17,8 @@ Noteworthy changes in version 1.24.0 (unrelease)
    easier and to allow enabling/disabling of keys (requires GnuPG 2.4.6).
    [T7239]
 
+ * New flag to re-encrypt OpenPGP data (requires GnuPG 2.5.1).  [T1825]
+
  * cpp: Provide information about designated revocation keys for a Key.
    [T7118]
 
@@ -49,6 +51,8 @@ Noteworthy changes in version 1.24.0 (unrelease)
  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  GPGME_ENCRYPT_FILE                      NEW.
  GPGME_SIG_MODE_FILE                     NEW.
+ GPGME_ENCRYPT_ADD_RECP                  NEW.
+ GPGME_ENCRYPT_CHG_RECP                  NEW.
  gpgme_key_t                             EXTENDED: New field 'revkeys'.
  gpgme_revocation_key_t                  NEW.
  gpgme_set_ctx_flag                      EXTENDED: New flag 'import-options'.
diff --git a/doc/gpgme.texi b/doc/gpgme.texi
index a630e0e2..0e64254b 100644
--- a/doc/gpgme.texi
+++ b/doc/gpgme.texi
@@ -6760,6 +6760,17 @@ output should be additionally encrypted symmetrically even
 if recipients are provided. This feature is only supported
 for the OpenPGP crypto engine.
 
+@item GPGME_ENCRYPT_ADD_RECP
+@item GPGME_ENCRYPT_CHG_RECP
+@since{1.24.0}
+
+Instead of encrypting, decrypt the input and write an output which is
+additionally encrypted to the specified keys.  The CHG flag is similar
+but does not add encryption to the specified keys but existing
+encryption keys by the new ones.  This feature is only supported for
+the OpenPGP crypto engine and requires at least GnuPG version 2.5.1.
+
+
 @item GPGME_ENCRYPT_THROW_KEYIDS
 @since{1.8.0}
 
diff --git a/src/engine-gpg.c b/src/engine-gpg.c
index fc9c7f90..eba61ac4 100644
--- a/src/engine-gpg.c
+++ b/src/engine-gpg.c
@@ -456,6 +456,22 @@ have_option_proc_all_sigs (engine_gpg_t gpg)
 }
 
 
+static int
+have_cmd_modify_recipients (engine_gpg_t gpg)
+{
+  static unsigned int flag;
+
+  if (flag)
+    ;
+  else if (have_gpg_version (gpg, "2.5.1"))
+    flag = 1|2;
+  else
+    flag = 1;
+
+  return !!(flag & 2);
+}
+
+
 static void
 free_argv (char **argv)
 {
@@ -2403,11 +2419,32 @@ gpg_encrypt (void *engine, gpgme_key_t recp[], const char *recpstring,
   if (gpg->flags.use_gpgtar && (flags & GPGME_ENCRYPT_WRAP))
     return gpg_error (GPG_ERR_INV_VALUE);
 
+  if ((flags & (GPGME_ENCRYPT_ADD_RECP|GPGME_ENCRYPT_CHG_RECP))
+      && !have_cmd_modify_recipients (gpg))
+    return gpg_error (GPG_ERR_NOT_SUPPORTED);
+
   if (recp || recpstring)
-    err = add_arg (gpg, "--encrypt");
+    {
+      if ((flags & GPGME_ENCRYPT_ADD_RECP))
+        err = add_arg (gpg, "--add-recipients");
+      else if ((flags & GPGME_ENCRYPT_CHG_RECP))
+        err = add_arg (gpg, "--change-recipients");
+      else
+        err = add_arg (gpg, "--encrypt");
+    }
 
   if (!err && ((flags & GPGME_ENCRYPT_SYMMETRIC) || (!recp && !recpstring)))
-    err = add_arg (gpg, "--symmetric");
+    {
+      if (!recp && !recpstring)
+        {
+          if ((flags & GPGME_ENCRYPT_ADD_RECP))
+            err = add_arg (gpg, "--add-recipients");
+          else if ((flags & GPGME_ENCRYPT_CHG_RECP))
+            err = add_arg (gpg, "--change-recipients");
+        }
+      if (!err)
+        err = add_arg (gpg, "--symmetric");
+    }
 
   if (!err && use_armor)
     err = add_gpg_arg (gpg, "--armor");
diff --git a/src/gpgme.h.in b/src/gpgme.h.in
index a01a24a6..cdd945bf 100644
--- a/src/gpgme.h.in
+++ b/src/gpgme.h.in
@@ -1356,7 +1356,9 @@ typedef enum
     GPGME_ENCRYPT_WRAP = 128,
     GPGME_ENCRYPT_WANT_ADDRESS = 256,
     GPGME_ENCRYPT_ARCHIVE = 512,
-    GPGME_ENCRYPT_FILE = 1024
+    GPGME_ENCRYPT_FILE = 1024,
+    GPGME_ENCRYPT_ADD_RECP = 2048,
+    GPGME_ENCRYPT_CHG_RECP = 4096
   }
 gpgme_encrypt_flags_t;
 
diff --git a/tests/run-encrypt.c b/tests/run-encrypt.c
index 8e8b559c..5a10e593 100644
--- a/tests/run-encrypt.c
+++ b/tests/run-encrypt.c
@@ -143,6 +143,8 @@ show_usage (int ex)
          "  --openpgp          use the OpenPGP protocol (default)\n"
          "  --cms              use the CMS protocol\n"
          "  --uiserver         use the UI server\n"
+         "  --add-recipients   use the re-encrypt feature\n"
+         "  --change-recipients  ditto, but clear existing keys\n"
          "  --loopback         use a loopback pinentry\n"
          "  --key NAME         encrypt to key NAME\n"
          "  --keystring NAMES  encrypt to ';' delimited NAMES\n"
@@ -240,6 +242,18 @@ main (int argc, char **argv)
           protocol = GPGME_PROTOCOL_UISERVER;
           argc--; argv++;
         }
+      else if (!strcmp (*argv, "--add-recipients"))
+        {
+          flags |= GPGME_ENCRYPT_ADD_RECP;
+          flags &= ~GPGME_ENCRYPT_CHG_RECP;
+          argc--; argv++;
+        }
+      else if (!strcmp (*argv, "--change-recipients"))
+        {
+          flags |= GPGME_ENCRYPT_CHG_RECP;
+          flags &= ~GPGME_ENCRYPT_ADD_RECP;
+          argc--; argv++;
+        }
       else if (!strcmp (*argv, "--key"))
         {
           argc--; argv++;